This invention relates to a method and a system for keeping information secret by the use of a cryptography technique and, in particular, to the method and the system for use in sharing a secret key between two remote systems. Herein, it is to be noted here throughout the instant specification that such a secret key is helpful as an absolutely or unconditionally safe cryptography key and is formed on the basis of a quantum cryptography protocol.
Heretofore, various protocols have been proposed as quantum key distribution protocols, for example, for cryptography using four non-orthogonal states, cryptography using two non-orthogonal states, cryptography using quantum interference between two photons, cryptography using time difference interference. Such protocols have been described in detail in Japanese Patent Unexamined Publication No.2000-286841 (Reference 1) and therefore will not been explained later.
Now, it should be noted that this invention is concerned with the quantum cryptography key distribution protocol using the four non-orthogonal states, among the above-enumerated protocols. The quantum cryptography key distribution protocol may be simply called the four state protocol BB84, as pointed out in Reference 1. The four state protocol BB84 would be implemented by a first way of using four non-orthogonal polarization states as the four non-orthogonal states or a second way of using four non-orthogonal states represented by superposition of quantum wave packets composed of a small number of photons having different time and space characteristics.
Both of the ways have been strongly studied and developed for practical use and have been mentioned in “Experimental Quantum Cryptography” written on page 120 et seq by Hugo Zbinden et al in a book entitled “INTRODUCTION TO QUANTUM COMPUTATION AND INFORMATION” (edited by Hoi-Kwong Lo et al and published by World Scientific in 1998) and also in “Quantum Cryptography” written on page 15 et seq by A. Ekert in the book “The Physics of Quantum Information (edited by D. Bouwmeester et al and published by Springer in 2000)”.
As regards the four state protocol BB84, the first way of using the four non-orthogonal polarization states is advantageous in that secure and equal bulk polarizers can be used in an encoder and a decoder included in a transmitter and a receiver, respectively. However, the first way needs a control operation of tracking polarization states on the transmitter and the receiver and makes it difficult to reduce a size of each device in the transmitter and the receiver because of using the bulk polarizers. Moreover, an optical loss inevitably becomes large when the first way is used.
On the other hand, the second way of using the four non-orthogonal states represented by superposition of the quantum wave packets composed of a small number of photons having the different time and space characteristics dispenses with polarization control and is capable of reducing an optical loss. In addition, the second way is advantageous in that wave-guide elements of small sizes can be used in an encoder and a decoder included in a transmitter and a receiver. However, the second way encounters a difficulty of structuring secure and equal interferometer systems, although this difficulty can be overcome by forming such interferometer system devices by solid-state monolithic devices.
Under the circumstances, vigorous studies have been continued about possibilities of both the first and the second ways in Europe, USA, and Domestic research and development organizations for the time being.
The following description will be made only about a conventional technique for carrying out the second way because this invention is concerned with the second way.
The conventional technique or system according to the second way is disadvantageous in that utilization efficiency of photons (may be called photon utilization efficiency) arriving at the receiver is reduced about to 0.5 and phase modulators should be included in both the transmitter and the receiver. As a result, an optical loss of 3 dB should be unconditionally caused to occur at the receiver on observation of the photons with time because of presence of the phase modulator included in the receiver.